DD .vs. OSB/drywall... Does that tiny extra bit of density that 5/8 drywall has really out weight the perks of a 3/4 OSB first layer?

I have spent the last couple of hours searching here on AVS and using the modern miracle that is google. Trying to determine why I would want to build my HT internal walls with 5/8" DD, over a combo of 3/4" (23/32) OSB and 5/8 drywall. (for sake of discussion lets assume both utilize GG )

Seeing as the vast majority of builds posted here that use a multi-layered leaf, also use drywall exclusively. I thought I'd pose the question as it's own thread so many can throw their two cents in without cluttering up individual members builds.

Here's my logic for giving OSB the nod:

In my particular room, I have loosely calculated that all four walls and ceiling will take 36 4x8 sheets to cover.

So a single layer of 5/8 drywall will cost roughly: $330cdn (w/o tax)

A single layer of 3/4 OSB will be in the ball park of: $442cdn (w/o tax)

Now sure, there's an additional $112 to the budget that doesn't need to be there. However, for all of those who hung their own drywall. Would it be worth $100 to you if you never needed to worry about hitting that stud while doing that second layer..? Or not needing to buy a box of extra long screws to reach those studs..?

I'm also weighing in the possible savings of going to 24" centers as compared to 16" for the wall construction. That in itself cuts the OSB's added expense by 50%. I'm also hoping (feel free to comment), that the use of OSB as a first layer, will give my true room in room build enough added structural strength as to eliminate the need of any ties/clips (eg: flanking paths) to surrounding structure for support.

So we have a hard number for OSB, at 40lbs/cu.ft. Using those tables, drywall only has about a 2lb advantage in weight per 4x8 sheet. However it is 1/8 thinner in dimension. Which makes me believe it does have more density. Unfortunately I don't possess the math skills to determine how much. Although, I can't really see it being all that much.

So after all my digging, the only positive I can come up with for the exclusive use of drywall is what I believe to be a slightly extra bit of density. How much of a benefit is that extra bit...? That's what I want to know.

If anyone in the know has some information as to why DD is the more supported construction methodology, please comment. I just can't believe that 1/8 difference in thickness trumps all the advantages a first layer of OSB provides.

Mass is mass. True enough. There is the other factor of stiffness also. If, as the OP indicated, GG is involved, there is the matter of damping efficiency. Damping is optimized when the stiffness of each layer is the same.

So double 5/8" drywall is massive and equal in stiffness. Optimal. Small deviation in stiffness (no idea what that delta is) will reduce damping efficiency.

Having said all that, I would myself be inclined to use an initial layer of OSB for the sheer convenience of being able to stick a screw anywhere layer. The small varience in mass and stiffness is likely quite minimal to the point of being a non-issue.

My ceiling is RSIC, 5/8" OSB, GG, 5/8" drywall. I did it for the convenience of having a continuous nailing surface for attaching the soffits. That convenience outweighed any sound isolation disadvantages for me.

My ceiling is RSIC, 5/8" OSB, GG, 5/8" drywall. I did it for the convenience of having a continuous nailing surface for attaching the soffits. That convenience outweighed any sound isolation disadvantages for me.

In all honesty... My first consideration is sound isolation with my build. So you would think it's a DD, GG approach for me. However, given what info I have been able to find (not much mind you), I think I'm going to side with convenience and use OSB as the first layer.

I was kinda hoping that some of the members with a build or two under their belt using DD construction, would chime in on why they opted for that method.

I've been thinking about it non-stop since my intial post, and the only other negative I've come up with is the extra effort it would take to cut holes for electrical boxes and HVAC hardware. ....and really, who here doesn't own a jigsaw..?

In all honesty... My first consideration is sound isolation with my build. So you would think it's a DD, GG approach for me. However, given what info I have been able to find (not much mind you), I think I'm going to side with convenience and use OSB as the first layer.

I was kinda hoping that some of the members with a build or two under their belt using DD construction, would chime in on why they opted for that method.

I've been thinking about it non-stop since my intial post, and the only other negative I've come up with is the extra effort it would take to cut holes for electrical boxes and HVAC hardware. ....and really, who here doesn't own a jigsaw..?

I hope I'm not missing something...

A plunge router will make short work of OSB. You may just have to use a more substantial bit than in a cutout tool. But I don't know; a cutout tool and bit may work fine, except for being a little slower cutting.

A trim router and straight cut bit would work perfectly. You just draw your box outline, trim it out, and hang it.

So we have a hard number for OSB, at 40lbs/cu.ft. Using those tables, drywall only has about a 2lb advantage in weight per 4x8 sheet. However it is 1/8 thinner in dimension. Which makes me believe it does have more density. Unfortunately I don't possess the math skills to determine how much. Although, I can't really see it being all that much.

Accoriding to the calculations, we get the following numbers:
3/4" OSB = 2.4375 Lbs per Square Foot (Lbs/Ft^2)
5/8" Drywall = 2.5 Lbs per Square Foot
These numbers could easily be used to calculate panel weights for those wishing to use a panel size other than 4x8.

The typical scientific unit of density is Kilograms per cubic meter (kg/m^3) and for that we get:
3/4" OSB = 653.704
5/8" Drywall = 767.6774

Just thought I would do the calculations for you since you mentioned it.

And more food for thought if you wish to take this any further, The MOE (Modulus of Elasticity) for OSB can be found here:http://www.fpl.fs.fed.us/documnts/pd...04_wang004.pdf
This document lists MOR (Modulus of Rupture) at 1.77-2.02 Mpa and MOE as 2.44-2.73 Mpa. (Both for 23/32" OSB)

I believe the document you linked to for Drywall also listed the MOR and MOE for that product.

This should theoretically give Ted his Delta for stiffness of OSB and Drywall I believe.

Given all the other great research here, I'm just wondering how you figured this...

Quote:

I'm also weighing in the possible savings of going to 24" centers as compared to 16" for the wall construction. That in itself cuts the OSB's added expense by 50%.

How does it cut the cost of the OSB by 50%? It will cut the cost of the lumber to build a wall, but not by 50%, more like 33%. If you build a 8X16ft wall, you still need 4 4x8 sheets regardless of the studs centering.

Rick- Thanks for the break down with the figures, and the links. I may just burn some time during my next shift working on that unknown delta.

Quote:

Originally Posted by tlogan6797

Given all the other great research here, I'm just wondering how you figured this...

How does it cut the cost of the OSB by 50%? It will cut the cost of the lumber to build a wall, but not by 50%, more like 33%. If you build a 8X16ft wall, you still need 4 4x8 sheets regardless of the studs centering.

Well... that was based on the build cost of my particular room Tom.

I don't have my material price/count sheet in front of me, but switching from a 16"oc to a 24"oc dropped my stud count by roughly 27. The cost of a 2x4x8 stud was just over $2cdn.

So that gives me a savings of $54. Which is just a hair less than 50% of the additional $112 the use of OSB for the first layer was going to cost me. ...compared to 5/8 drywall that is.

Another consideration is the ease of cutting drywall vs OSB, not only for the outlets as discussed, but for all cuts. Drywall is a simple score-break-cut, whereas OSB you gotta pull out the table saw, use a track saw, or a skill saw combined with a jig of some sort. Unless your walls are perfect multiples of 4' and 8', this could get very annoying very fast and quickly outweigh the benefit of not having to worry about finding a stud on the second layer. Especially if you have a bunch of little jut-outs, or odd shaped walls or something.

The room that is to become my home theater was originally planned to be a combo gym/tornado shelter (we recently had a couple of close calls, so my wife and kids really wanted a shelter). It was going to be the first room that I finished, so I had already purchased 7/16 OSB and had planned to use 2 layers of it.

Now that it is a HT instead, we don't really have a good shelter room, but, since I already have the OSB, I'm probably going to still use the OSB and a single layer of drywall for my HT. While a "room within a room" probably makes a horrible shelter, at least the OSB should keep flying debris out (assuming the room doesn't get picked up and carried off, lol).

Anyway, I really like the idea of using the OSB because of the already mentioned benefits, plus it allows us the opportunity to have a room to go to during a storm.

One potential negative is that it would burn a lot easier than drywall, though.

That being said, every wall in my house is covered on the outside by OSB between the studs and concrete siding, so I'm not sure if there's a big difference between that and covering the interior walls with OSB. Maybe it's different stuff, I dunno.

Regarding Ted's point, that OSB will result in less then ideal performance from GG, what about OSB + Dry wall + GG + Dry Wall? Or does the whole wall need to "flex" at the same rate for GG to work an intended?...cost and wall thickness aside

Ted - any thoughts on how much performance you'd be giving up for using OSB+GG+Dry wall? I suppose it depends on stud spacing and a whole heap of other things?

Well, I recently went through the same dillema with my garage (except for the acoustic part of it, of course) I went between putting up sheetrock or OSB for a while and I finally settled on OSB.

I decided that (being a garage/shop) it would be great to just be able to attach things to the walls wherever I needed to. I was worried about the installation, but I went for it anyway. Being a garage/shop I put in outlets every 3rd stud, so I had to make a lot of cutouts, but with a jigsaw and roto tool it really wasn't much worse than cutting 'rock.

I did take more time to make sure I was accurate with my cuts, but it still went pretty quick. The harder part was ripping the sheets down with my circular saw and straight edge, even that wasn't too bad.

In the end it was well worth it to be able to hang shelves or whatever where ever I needed. I think it would be the same when you go to put the 'rock over it. Just being able to put screws in indiscriminately will make it go pretty quick.

Just for reference, my garage is 22' x 26' x 13' (DxWxH) and I used 7/16" OSB. A little thinner than what you will use, but shouldn't make much difference.

Regarding Ted's point, that OSB will result in less then ideal performance from GG, what about OSB + Dry wall + GG + Dry Wall? Or does the whole wall need to "flex" at the same rate for GG to work an intended?...cost and wall thickness aside

You would want GG between all of the layers.

Ted - any thoughts on how much performance you'd be giving up for using OSB+GG+Dry wall? I suppose it depends on stud spacing and a whole heap of other things?

Other factors are certainly in play but pretending for a moment that the drywall or OSB weigh the same, the difference in damping comparing OSB+DW and DW+DW would be quite small

Actually, her exact words were that there isn't a fire code for this per se...but that it does significantly reduce the burn time for walls constructed this way (ie: they burn faster).

I really hope that you didn't intend to post a sarcastic reply because you were the one asking for help.

Didn't mean to come off sarcastic...sorry about that. However I would really be surprised if the use of OSB on a wall that is framed with wood already, ends up breaking a fire code.

I have been involved in commercial construction (not a HT), where the mandatory fire rating was/is much higher. To the extent to which wooden reinforcement of steel stud for hanging cabinets and such, was against fire code. That's why I commented that it wasn't commercial we are discussing.

i know this is a old thread but was curious has feelings changed towards this approach? it seems the more theater builds i see there is a decent percentage of folks using 5/8 osb for there first layer and the GG and 5/8 drywall
as opposed to two layers of 5/8 drywall and GG... is there any real advantage to this approach and for a new Home theater builder what are some thoughts?

i know this is a old thread but was curious has feelings changed towards this approach? it seems the more theater builds i see there is a decent percentage of folks using 5/8 osb for there first layer and the GG and 5/8 drywall
as opposed to two layers of 5/8 drywall and GG... is there any real advantage to this approach and for a new Home theater builder what are some thoughts?

thanks

I did mine with OSB/GG/Drywall (one layer) for two reasons. The main reason was because I didn't trust myself to get every stud at exactly 16" on center and hanging one layer of drywall may not be too bad to work around that but the second layer would have resulted in me poking lots of holes "hunting" for a stud.

The second reason was because I built my soffit after the walls went up and I plan to do fabric frames so in both instances, I just sink a screw into the wall and life goes on. It literally removed ALL of the annoyances of hanging drywall.

Unfortunately, you still have to tape, mud, and sand and that's infinitely more annoying than any other drywall-related task. I'm getting cold sweats just thinking about that part of my build... Eeeck.

Also, Ted has approved this method for Green Glue discounting the difference in density as negligible.

On the downside, I didn't take my riser into account on one of my outlet heights during rough in and I'm not moving it now (it's about 6" above the riser instead of 16"). I tell myself it's because it'll be behind a fabric panel but truthfully it's because I'd need a sawzaw to cut through the OSB layer to raise it up and I'm far too lazy for that.

Check out one of Big's builds (the Ohio one, I believe) for an example of an OSB/Drywall room.

I don't have a build thread because maintaining one would take as much time as building the room and my wife thinks I'm slow enough as it is.

I like the idea of OSB-GG-DW myself as well, hanging that rock will be much much easier, as well as the soffits I will have to build around ductwork(on 1 side- matching on the other).
I'm pretty certain this is the route I am going to take when I start building in the early spring.

Thanks for the thread and the info guys.

Why waste $ on more cheap stuff, it's like challenging a dragon with a pocket knife.

Working with OSB is a PIA, takes longer to cut, you get splinters in your hands. and the drywall screws heads won't sink completely in the board so you need to be careful not to spin out the threads of the screw in the metal channel. But I did it in one theater by request of the client and it was already sitting in the basement (Ohio Build), and on another occasion where we planned to add an elaborate ceiling treatment. (see "rescue my theater") Don't think that having OSB on the ceiling gives you a free pass on screwing up the second layer of drywall haphazardly. If you don't make an effort to hit the channel all the weight of the ceiling is on the screws for the first layer. The advantage of a two layer ceiling or wall is that you don't have to get your seams to align with the middle of a channel. This is a real time saver.

You can do just the ceiling if you are planning an elaborate treatment and then do two layers of DW on the walls.

Be aware that there are theater designers who recommend against using OSB as the first layer and using different structures from the ceiling to walls for room calibration reasons.